Many patents disclose conditions for converting aldehydes to acetals. For example, U.S. Pat. No. 2,131,998 discloses the addition of an alcohol to a double bond as well as acetal formation, i.e., ##STR1## U.S. Pat. No. 2,566,559 discloses the use of a fixed bed cation exchange resin as a catalyst for the preparation of acetals and U.S. Pat. No. 2,840,615 discloses the use of strongly acidic cation exchange resins as catalysts for acetal formation from methanol and acetaldehyde. U.S. Pat. No. 2,678,950 discloses the use of sulfo acid catalysts with continuous removal of water from the reaction mixture and U.S. Pat. No. 3,014,924 discloses the use of highly porous silica-alumina catalysts impregnated with a small quantity of a strong mineral acid. U.S. Pat. No. 2,987,524 discloses the preparation of cyclic unsaturated acetals using a sulfo acid catalyst with continuous removal of water. U.S. Pat. No. 2,729,650 discloses the preparation of unsaturated cyclic acetals using inorganic salts as catalysts.
The reactions of alcohols with aldehydes to form acetals are equilibrium reactions. The degree of conversion to the acetal is limited by the equilibrium constant for the reaction, unless one of the products can be removed from the reaction site. ##STR2##
K is the equilibrium constant, and the various C's represent the molar concentrations of reactants and products. The equilibrium constant for the reaction of methanol with acetaldehyde allows only about a 50% conversion to acetal while the reaction of acrolein with 2-methyl-1,3-propanediol (MPD) yields only 65% of acetal under equilibrium conditions.
Several techniques have been used in an attempt to obtain conversions of reactants to acetals at concentrations higher than the equilibrium concentration. The most common technique used is the completion of the reaction by azeotropic distillation of water with a water-insoluble organic solvent such as benzene or toluene as disclosed in U.S. Pat. No. 2,987,524. Such a process suffers from several basic deficiencies. The overall efficiency is low because low yields of acetal are obtained per volume of reactor space, a high energy consumption is needed for the azeotropic distillation, acetal product must be separated by distillation from the solvent, the cost of solvent adds to the process cost and at the temperatures and times required for azeotropic distillation, unsaturated aldehydes, such as acrolein, react to form side products by polymerization and addition of water and alcohols to the carbon-carbon double bond.
Large excesses of one reactant, usually the alcohol, have been used to drive the equilibrium toward higher conversions with more complete utilization of the aldehyde. U.S. Pat. No. 2,566,559 teaches the preferred use of 4 to 5 moles of alcohol per mole of aldehyde. The acetal product must be separated from the large excess of alcohol, and the alcohol recovered and recycled to the process. High molar ratios of aldehyde/alcohol may be used, but side polymerization reactions and additions to the double bond may consume some of the unsaturated aldehydes.
Water has also been removed from the reaction by dessicants, such as calcium chloride as disclosed in German Patent 434,989. These systems are difficult to handle and costly to operate, because the dessicant must be recovered, dried and returned to the process.